1. Field of the Invention
The invention relates to well testing of hydrocarbon reservoirs to determine economic viability.
The purpose of reservoir simulation is to determine as precisely as possible the extent (volume), nature, permeability, and porosity of the payrock.
2. Prior Art Discussion
In well testing a wellbore is drilled into the payrock, usually at an angle to vertical. The wellbore is lined and the lining is perforated at locations within the payrock. Oil or gas in the payrock flows into the wellbore through these perforations and the pressure arising from his flow is measured by pressure gauges within the wellbore. Flow of oil or gas from the wellbore opening is controlled by pumps and valves at the opening.
For simulation, the hydrocarbon stock which flows from the wellbore is analysed and parameters such as the compressibility and the viscosity are determined. Also, geological surveys are performed. The combined information so gathered is used to estimate the payrock properties. These properties are used by a simulation tool to estimate the pressure curve (as a function of time). The estimated curve is fed back to change the input payrock properties in an iterative manner until the estimated pressure curve matches closely the actual measured curve. The particular payrock properties for this iteration stage should be reasonably accurate.
While this method is quite sound in its reasoning, it suffers from a major drawback. This is an inaccuracy which arises because of use of crude representations of the payrock geometry and material properties. If the geometry and material distribution data is very inaccurate, the overall analysis is generally compromised.
The invention is therefore directed towards addressing this problem by providing for more accurate simulation with less engineer time requirement.
According to the invention, there is provided a hydrocarbon reservoir analysis method comprising the steps of simulating hydrocarbon flow from the reservoir into a wellbore and analysing simulated wellface pressure response by comparing it with measured pressure data, characterised in that the reservoir is modelled before simulation as a solid model comprising polygons in plan and layers in elevation, finite elements are generated in patterns in the polygons and the layers to provide a mesh, and simulation is performed with said mesh.
In one embodiment, the method comprises the further step of selecting an appropriate template model from a set of template models and modifying the selected model.
In one embodiment, the selected model is modified by changing the numbers of layers and the shapes of the polygons.
In one embodiment, the polygon shapes are modified by changing locations of control points at polygon corners and the number of layers is changed by changing depth data associated with said control points.
In one embodiment, the model is represented by objects instantiated from classes.
In one embodiment, the model is represented by:
a shape object defining the overall reservoir shape;
a polygon object defining each polygon in terms of an aerial region in plan bounded by edges defining vertical planes; and
a layer object defining each layer in terms of the bounding planes above and below.
In one embodiment, the mesh is generated by generating a pattern object defining elements extending in an elevational plane.
In one embodiment, a pattern object defines elements in a plane extending radially from the wellbore for a wellbore polygon.
In one embodiment, the plane extends from the wellbore to the polygon edges.
In one embodiment, the pattern object defines progressively fewer elements as it extends from the wellbore.
In another embodiment, the pattern object is swept rotationally from a starting plane extending radially from the wellbore to fill the polygon containing the wellbore.
In one embodiment, a pattern object is swept translationally from a starting plane corresponding to a generator line and defines elements in a direction extending from the generator line in the direction of an adjoining base line.
In one embodiment, the base line and the generator line coincide with polygon boundaries.
In one embodiment, the base and generator lines are defined as such in the shape object, and each pattern object is related to the polygon objects and the shape object according to a condition that each polygon comprises at least one base line and at least two generator lines.
In one embodiment, the pattern objects are inter-related in a manner whereby they are ranked according to their relationship with the wellbore polygon.
In one embodiment, the wellbore polygon has a first rank level, polygons adjoining the wellbore polygon have a second rank level, polygons adjoining the second rank polygons have a third rank level, and subsequent polygons are ranked accordingly.
In one embodiment, each pattern object defines elements according to facets linking layer bounding planes.
In one embodiment, the simulation is performed according to algorithms which inextricably couple finite element mesh generation, material property assignment. and equation solving.
In one embodiment, variable precedence data required for equation solution is inferred and constructed within mesh generation.
In one embodiment, the simulation imposes boundary conditions on parts of the wellbore, leading to a set of pressure equality constraints used to re-map the precedence data to reduce computation time.
In one embodiment, the simulation step comprises the sub-steps of representing time step history, minimum dimensionless pressure, and maximum dimensionless pressure as lines in a pressure/time graph providing controls for a colour range, and receiving input instructions in the form of movement of said lines to a desired position.
According to another aspect, the invention provides a hydrocarbon reservoir analysis system comprising means for performing a method as defined above.